Abstract
Annual growth patterns in marine mollusc shells are valuable indicators of the condition of marine ecology through time. In archaeological contexts, the mollusc’s time of death (i.e. the last season of growth) is an indicator of human exploitation patterns throughout the year, enabling the reconstruction of when and how often gathering occurred as well as when sites were occupied. Both pieces of information, growth rate and season of death, are vital for understanding exploitation pressure(s) in the past, and building baselines for modern environmental policies that secure sustainable marine resources. Previously, these parameters have been determined by incremental growth-line or isotopic analyses, which are time consuming and often expensive techniques, thus restricting sample size and the overall robustness of palaeoecological interpretations. Here, we apply Laser Induced Breakdown Spectroscopy (LIBS) to produce elemental maps (Mg/Ca) with the potential to trace and display growth patterns quickly, and at a reduced cost. We further compare the elemental maps with the results obtained from incremental growth-line analysis to provide a structural context for the geochemical data, and demonstrate the utility of an integrated methodological approach. Our pilot study was undertaken on 12 European oysters (Ostrea edulis, Linnaeus, 1758) from the Late Mesolithic shell midden at Conors Island, Co. Sligo in the Republic of Ireland. Our LIBS analysis enabled us to accurately and quickly determine repeating growth patterns, which were often in agreement with the annual growth increments visible through the microscopic analysis. Based on this comparative dataset, including structural and geochemical patterns, the Late Mesolithic site of Conors Island had been occupied throughout the year. Moreover, our analyses highlight the applicability of LIBS to determine prehistoric seasonality practices as well as biological age and growth at an improved rate and reduced cost than was previously achievable.
Highlights
IntroductionOstreidae (oysters) are a family for which growth data are frequently needed. information on growth rates and population dynamics of Ostrea edulis (European oyster, Linnaeus, 1758) are important for the conservation, restoration, and management of modern (Kraeuter et al 2007; Powell et al 2008; Harding et al 2010; Levinton et al 2013; Baggett et al 2015), and past populations (Rick and Lockwood 2013; Rick et al 2016, 2017; Kusnerik et al 2018).Ecosystems from a range of periods have been assessed through incremental growth-line analyses, including prehistoric as well as pre-industrial or modern datasets, giving detailed information on marine ecosystems, habitats or the ecological behaviour of early hunter-gatherer-fisher communities (Kirby and Miller 2005; Blitz et al 2014; Rick et al 2016, 2017)
Ostreidae are a family for which growth data are frequently needed
Two annual patterns occurred in the majority of the specimens (Figure 5): (1) a gradual increase and subsequent decrease of Mg/Ca ratios across whole growth increments is in agreement with earlier conclusions that Mg/Ca ratios correlate with changes in growth rate (Figure 5a) (e.g. Durham et al 2017), and (2) a thin but distinct line with high Mg/Ca ratios consistent with annual growth lines during a growth stop (Figure 6b) (Schöne et al 2013)
Summary
Ostreidae (oysters) are a family for which growth data are frequently needed. information on growth rates and population dynamics of Ostrea edulis (European oyster, Linnaeus, 1758) are important for the conservation, restoration, and management of modern (Kraeuter et al 2007; Powell et al 2008; Harding et al 2010; Levinton et al 2013; Baggett et al 2015), and past populations (Rick and Lockwood 2013; Rick et al 2016, 2017; Kusnerik et al 2018).Ecosystems from a range of periods have been assessed through incremental growth-line analyses, including prehistoric as well as pre-industrial or modern datasets, giving detailed information on marine ecosystems, habitats or the ecological behaviour of early hunter-gatherer-fisher communities (Kirby and Miller 2005; Blitz et al 2014; Rick et al 2016, 2017). One way of contextualising irregular shell growth in oysters are geochemical proxies for seasonally occurring environmental changes (see Surge et al 2001; Lulewicz et al 2018; Zimmt et al 2019); the most established proxy being δ18O isotope analysis, which in shell carbonates primarily reflects the δ18O values of the immediate environment and temperature during the calcification process (Epstein et al 1953; Grossman and Ku 1986). This approach requires a general understanding of the growth structure, a sampling resolution that appropriately covers this structure to provide a seasonal resolution (West et al 2018), and lastly expensive mass spectrometric analysis
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